1. Field of the Invention
The present invention relates to a system for determining the current-voltage characteristics of photovoltaic array, and particularly for such a system that facilitates the automatic plotting of the current-voltage characteristic curve of such an array at a selectable illumination level.
2. Description of the Prior Art
In the manufacture and testing of silicon photovoltaic arrays such as solar panels and solar power modules, it is desirable to measure and plot the current-voltage characteristics of each array. Several problems are encountered when this is done automatically.
First, it is important to provide uniform illumination over the entire solar module surface. This requires the use of a high intensity light source placed at a sufficient distance from the device under test so that the illuminating rays are parallel over the entire array surface. This is best accomplished using a high power xenon or like flash tube. Typically such flash tube is excited by an R-C circuit so as to produce repetitive flashes at a fixed duty cycle, usually synchronous with the 60 Hz power line frequency. By placing such a flash tube at say 7 feet from the device under test, substantially parallel light beam illumination is obtained over the entire array surface of a typical array which may be from two feet square to about five feet square.
With such an arrangement, a typical peak light output value at the surface of the photovoltaic array may be on the order of 125 mW/cm.sup.2. However, since the illumination is provided from a flash tube, the actual light occurs as a series of flashes during each of which the light level rises from zero to the peak value and then falls back to zero. If the output characteristics of the photovoltaic array are to be measured at a known light intensity level, provision must be made to take the measurement only when the incident light level is at the desired value. One object of the present invention is to provide such a system wherein, while using a flash tube illumination source, the array output characteristics can be measured automatically at any desired illumination level up to the peak output value of the flash lamp source.
A second problem arises from the current-voltage characteristics themselves. This can be understood in conjunction with FIG. 2 which shows a plot of the output current as a function of output voltage from a typical photovoltaic array.
Under approximately short-circuit or very low resistance load conditions, the array provides a relatively high current output. With a very slight change in load resistance, the array output voltage may change rapidly from about 2 volts to about 20 volts with a minimal change in current. In this "current mode" region, accurate measurement requires changing the value of the low resistance load in very small increments. If this is not done, the measurement points will change abruptly from say 2 volts to say 20 volts, with few, or possibly no measurements obtained at intermediate voltage levels. With such an arrangement, an associated X-Y plotter will jump directly from a starting position of low voltage to a second position of high voltage, without accurate plotting of the intermediate points. In other words, the plotter would only indicate the points at the beginning and end of the current mode region of the output characteristic curve. Inaccuracy would result.
As the load resistance increases, the photovoltaic array output exhibits a "voltage mode". As can be seen in FIG. 2, in this mode the current changes significantly with relatively smaller change in output voltage. For accurate plotting of this portion of the curve, it is desirable to take consecutive measurements with relatively larger incremental changes in the load impedance.
Another object of the present invention is to provide a system for changing the load impedance across the photovoltaic array under test in a nonlinear fashion, so that in the current mode the load resistance is changed in small increments, and under voltage mode conditions the load resistance is changed in larger increments.
Other objects of the present invention are to provide a system for determining the current-voltage characteristics of a photovoltaic array wherein:
1. The illumination level at which the measurements are taken is selectable;
2. Each measurement is taken when the instantaneous illumination level is at the desired value, by using appropriate sample and hold circuits; 3. Nonlinear load circuitry is employed which uses the transfer characteristics of a field effect transistor (FET) to provide the necessary nonlinear load characteristics for the array under test;
4. Ramp generator circuitry is used in conjunction with the nonlinear load circuit to achieve consecutive load conditions that will result in measurement on the entire current-voltage characteristic curve of the array under test, thereby facilitating;
5. The use of a conventional X-Y plotter automatically to produce an output characteristic curve for the device.